Self-assembled networks of conjugated oligomers.
Date of Issue2013
School of Materials Science and Engineering
2D self-assembly of non-covalently bonded structures offers an attractive way to prepare processable functional organic macromolecular materials and offers a lot of prospective ways for making progress in nanoscience and advancing nanotechnology. Non-covalent interactions for example hydrogen bonding and metal ion co-ordination offer stability, and more importantly flexibility to the resulting networks. In this thesis work, novel, non-covalently bonded 2D supramolecular structures (trigonal and linear conjugated molecules facilitated via hydrogen bonding interactions and metal-ion co-ordination) have been studied. As a part of ongoing efforts into the development of self-assembled functional materials, this thesis work reports on the synthesis and characterization of conjugated oligomers bearing various functional groups, and some initial studies on their self-assembly using scanning tunneling microscopy (STM) and optical spectroscopy. Various 2D supramolecular networks of trigonal oligophenylenes were observed by STM at the liquid-graphite interface. Depending on whether the solvent used was either alkylated (n-tetradecane, phenyl-octane) or non-alkylated (tetrahydro-naphthalene), different types of networks were achieved. These periodic networks were either open networks (hexagonal arrangement with a central cavity) or close-packed structures. In the open network observed using tetrahydronaphthalene as the STM solvent the central cavity is occupied by several solvent molecules. Furthermore, in most cases it was observed that these arrangements were chiral, with left and right handed domains in the same sample. It was found that the nature of the solvent and time greatly influenced the self-assembly process as shown by both STM and photophysical studies on these molecules. Synthesis and self-assembly of linear and trigonal terpyridines (tpys) and 2D self-assembled networks of trigonal tpys with linear tpys through zinc metal ion (Zn(II)) co-ordination are also reported. The resulting Zn(II) co-ordination networks were characterized by means of X-ray photoelectron spectroscopy (XPS), small angle X-ray scattering (SAXS), BET, and other photophysical and electrochemical methods. These metallo-supramolecular Zn(II) networks revealed the influence of the metal ion on the thermal, optical and electrical properties of the synthesized metallo-supramolecular assemblies. Moreover, these networks have shown high luminescence with a long fluorescence life time and good thermal stabilities. Non-covalent interactions for instance hydrogen bonding and metal-ligand co-ordination can direct the intermolecular energy transfer process by the self-assembly of donor and acceptor molecules. Looking forward for the possibilities of these self-assembled networks for light harvesting applications, efficient intermolecular energy transfer was observed in the self-assembled network of trigonal amine molecules and linear PDI molecules. These new donor (D) and acceptor (A), D-A couples may serve as models for light harvesting systems. Apart from that, the monolayer patterns of these kinds of 2D networks could be used as a template to build hierarchical nanostructures. These hierarchical nanostructures may be tested in electronic devices and as templates for formation of hybrid organic-inorganic nanomaterials in future.